Apple - Open a specifc URL in Fluid.app
I had the same problem and wrote a little AppleScript to handle this. You can find the app here: https://github.com/biafra23/Default-Fluids
You need to make this script your default browser. This can be achieved with Safari: Menu -> Safari -> Preferences -> General -> Default web browser
The source is in https://github.com/biafra23/Default-Fluids/blob/master/default_browser.app/Contents/Resources/Scripts/main.scpt
It contains sections like this:
if this_URL contains "facebook.com" then tell application "/Applications/Fluids/facebook.app" activate open location this_URL end tell else if ... else -- default browser here tell application "/Applications/Firefox.app" activate open location this_URL end tell end if
You need to make changes to those according to your setup with the AppleScript Editor.
It is not as convenient as choosyosx but its free.
Developer of Fluid here. The only way I know of to do this is with an additional utility like Choosy or Browser Fairy (app store link).